Research on food webs increasingly relies on sampling biomarkers (stable isotopes and fatty acids) in consumers and their potential prey. In studies of macroalgal and seagrass biomarkers in the northeast Pacific, Dethier and coworkers found substantial variation in biomarkers across dates and sites. This natural variation means that incorrect conclusions may be drawn from assumptions that a particular biomarker value corresponds to a given food item in a consumer’s diet. Biomarkers are relatively consistent at the phylum level, but spatial and temporal variation at the species level is fairly high. The importance of this ‘noise’ in biomarker values was tested by running Bayesian mixing models with a theoretical consumer given diets whose biomarker values varied at scales seen in nature.